The present invention relates to an exhaust system for an internal combustion engine, in particular to an exhaust system employing a catalytic device for purifying the exhaust gases. The invention is especially suitable for a system for a lean burn engine (employing a lean NOx catalytic device), but it is not limited exclusively to this.
In general terms, the need to operate a catalytic device above a minimum operating temperature is well known in the art. For example, EP-A-0460507, GB-A2278068 and WO 96/27734 describe arrangements for routing the exhaust along an appropriate exhaust path if the gas is not at an optimum high temperature, or if the catalytic devices have not yet reached there optimum temperatures.
The increasing cost of fuel and the concern over CO2 emissions has lead a drive for engines with improved fuel economy. Lean burn engines have been developed using gasoline direct injection and port injection techniques.
Under these lean operating conditions the standard 3-way catalyst is very efficient for CO and hydrocarbon (HC) oxidation, but the reduction of oxides of nitrogen NOx (NO and NO2) to di-nitrogen (N2) is considerably more difficult. Catalytic converters and traps are being developed which can operate under lean conditions. The xe2x80x9cleanxe2x80x9d problem is that, there is generally an insufficient quantity of hydrocarbons in the exhaust gas to enable efficient conversion of all of the NOx to di-nitrogen at the catalytic device. One type lean burn engine uses a lean cycle and an intermittent stoichiometric or rich cycle. A catalytic trap can be used which absorbs the excess NOx gases during the lean cycle, and then converts the NOx to N2 in the presence of more hydrocarbons during the rich cycle. The rich cycle is sometimes referred to as the xe2x80x9cpurgexe2x80x9d cycle.
Although lean NOx catalytic converters and traps offer potentially enormous emissions benefits, it has been extremely difficult to attain the full potential of the catalytic devices, especially under conditions in which the engine is working hard (for example, for high speed vehicle cruising). The reason is that, under such conditions, the temperature of the exhaust gas entering the catalytic trap often exceeds the optimum operating range for the catalytic device. For example, FIG. 17 illustrates the typical temperatures characteristics for a lean NOx trap. The catalytic material has a coating for absorbing the excess NOx, but this is only effective up to about 450xc2x0 C. On the other hand, the reduction of the oxides in the presence of hydrocarbons is only effective at temperatures above about 200xc2x0 C. This creates a useful temperature window from approximately 200-450xc2x0 C. in which the lean NOx conversion can occur. At temperatures outside this window (for example, caused by high engine speed), the catalytic trap will not operate efficiently. Lean NOx catalytic converters also operate in a similar temperature range.
Broadly speaking, one aspect of the present invention is to provide a cooling heat exchanger unit upstream of a catalytic device, and a control device for providing selective cooling of the exhaust gas upstream of the catalytic device, using the heat exchanger.
With the invention, the heat exchanger unit can provide sufficient cooling to cool the hot exhaust gases to a desired catalytic operating temperature, or to within a desired operating temperature window, for efficient catalytic operation.
Moreover, cooling of the exhaust gases provides other performance advantages, specifically by reducing the volume of the gas, and thus the volume flow rate through the exhaust system. This can help reduce the backpressure within the exhaust system, and can also help reduce flow noise through the system, especially at high engine speeds and loads. These are significant problems associated with lean NOx catalytic devices, which tend to require relatively large substrates for efficient lean NOx operation. The use of large substrates can cause undesirable backpressure build up. The reduction in back pressure will help to improve fuel economy and reduce CO2 emissions.
The heat exchanger unit may be a gas cooled unit (for example, air cooled), or it may be liquid cooled. The latter is preferred for the following reasons:
(a) A liquid-cooled heat exchanger can avoid the occurrence of transient temperature drops which air-cooled exchangers can cause. Initially, an air-cooled heat exchanger will be much colder than the hot exhaust gases and, when the hot gases first pass through the exchanger, the large temperature difference causes a very efficient heatsink effect to occur. Such large transients can cause the temperature to fall below an optimum operating range of the catalytic device until the heat exchanger heats up to near the exhaust gas temperature;
(b) A liquid-cooled heat exchanger remains at the temperature of the coolant, and never heats up to the exhaust gas temperature. Heat transfer is achieved through the large heat capacity of the liquid, and does not depend (at least to much extent) on the precise temperature of the coolant itself. In contrast, an air-cooled exchanger necessarily heats up to near the exhaust gas temperature, and dissipates heat by being much hotter than the surroundings. This can cause design problems for placement on a vehicle away from hazardous (temperature sensitive) areas, and also requires the presence of a cooling air flow, in use.
(c) A liquid-cooled heat exchanger can enable the use of an open-loop control system for controlling the cooling operation without having to measure directly the temperature of the exhaust gas in the exhaust system. Most vehicles are not equipped with an exhaust temperature sensor, and the addition of such a sensor able to withstand harsh exhaust conditions represents additional expense. With a liquid-cooled system, the exhaust gas temperature can be predicted using the outputs from conventional vehicle sensors for sensing, for example, the engine inlet air temperature, the engine coolant temperature, the engine speed, the air mass flow entering the engine, and the fuel:air mixture (measured using a lambda sensor).
(d) A liquid heat exchanger can generally be made more compact than a air-cooled heat exchanger.
If a liquid heat exchanger is used, then preferably, this is coupled to an existing coolant circuit of a vehicle, such as, for example, the engine coolant circuit.
If a gas-cooled heat exchanger is used, then the arrangement should comprise a gas inlet tube, a heat exchanger unit coupled to the inlet tube, and an outlet tube exiting the heat exchanger unit, the heat exchanger unit having a greater heat dissipation effect than the inlet and outlet tubes.
In either type of system, the exhaust system preferably comprises a first flow path through the heat exchanger for cooling the gas in the first path, and a second flow path bypassing the heat exchanger. The second path may flow through the housing of the heat exchanger along a substantially non-heat exchange (or at least a low-heat exchange) path.
In another broad aspect, the invention provides a method, and also a control apparatus, for controlling operation of a cooling device for cooling exhaust gas upstream of a catalytic exhaust purification device.
In one preferred aspect, the method includes predicting the exhaust gas temperature from a plurality of characteristics which are each not directly indicative of the exhaust temperature, and controlling cooling operation in response to the predicted exhaust gas temperature.
In another preferred aspect, the method includes controlling the cooling during a first engine cycle to achieve an exhaust temperature within a first operating range for the catalytic device, and during a second engine cycle to achieve an exhaust temperature within a second operating range for the catalytic device.
The second operating range (achieved after the first operating range) may include a higher maximum temperature than the first operating range. For example, the second operating range may correspond to a stoichiometric cycle, or to a sulphur purge cycle. The first cycle may correspond to a lean cycle.